feat: add INatsClient interface and implement on NatsClient

Extract INatsClient interface from NatsClient to enable internal clients
(SYSTEM, ACCOUNT) to participate in the subscription system without
requiring a socket connection. Change Subscription.Client from concrete
NatsClient to INatsClient, keeping IMessageRouter and RemoveClient using
the concrete type since only socket clients need those paths.

src/NATS.Server/Subscriptions/Subscription.cs

@@ -9,5 +9,5 @@ public sealed class Subscription
     public required string Sid { get; init; }
     public long MessageCount;  // Interlocked
     public long MaxMessages;   // 0 = unlimited
-    public NatsClient? Client { get; set; }
+    public INatsClient? Client { get; set; }
 }